This invention relates to a recycle process for effectively and completely removing heavy metals from aqueous solutions with iron particles. More particularly, this invention relates to such a process wherein heavy metals are reclaimed, hazardous sludge is substantially reduced or eliminated and an effluent having very low biological toxicity is produced. The present invention is useful for treating metal ion containing waste water generated by industries such as metal plating, metal surface finishing or printed circuit board manufacturing.
Prior to the present invention, it has been generally accepted that plating waste metals removed from alkaline solutions as metal hydroxide sludges must be handled as hazardous waste. Environmental Protection Agency (EPA) jurisdiction over these wastes is well established. When generated by an electroplating facility and shipped off site, such sludge materials are defined as categorical F006 hazardous waste. Transporting and receiving and processing of these materials even for reclamation and recycling, are restricted to EPA or State licensed operators. Due partly to this, and also due in part to the low metal concentration in such sludges, high recycling costs are incurred that usually exceed the recoverable value of the metals.
Metallic iron has long been known to react directly with certain other metals that are dissolved in acidic aqueous solution. The iron dissolves into the acidic solution and the other dissolved metal deposits a metallic layer on the surface of the iron. Referred to as metallic replacement or cementation, this characteristic of metals has commonly been used in the commercial extraction of copper from ores and acid leaching of mine tailings. After some time, the surface of the iron is so covered with other metal that the iron becomes unreactive and the reaction ceases.
U.S. Pat. No. 3,902,896 addresses this limitation and discloses the use of a soluble thiosulfate compound to aid the cementation of such metals as copper, silver, gold and platinum group metals from aqueous solutions. The patent discloses that the cemented metal flakes off the base metal, exposing fresh surfaces. Two properties of thiosulfate limit its utility for this purpose. In strong acid solutions, thiosulfate decomposes to sulfur dioxide and elemental sulfur, which is colloidal and coats all surfaces it contacts. Also, dilute thiosulfate solutions are very corrosive on ferrous alloys, particularly on stainless steel materials.
U.S. Pat. No. 3,634,071 describes the use of sulfur dioxide for reducing ferric ions contained in recirculated ore leaching acid solutions. Some improvements in the cementation of copper using metallic iron were observed as relating to decreased oxidation of the iron and copper metals by ferric ions.
U.K. Patent Application GB 125828 A, filed Jun. 16, 1983 discloses a process for removing copper ion from solution by contacting the solution with steel wool, converting only a small portion of the iron into copper. This process is commercially undesirable due to 1) the uneconomically low conversion of iron to copper, and 2) the high cost of steel wool and 3) the high labor cost for handling the materials. The recovered copper has a lower recycling value due to the cost of processing required for separating it from the residual steel wool fibers.
Many other methods exist for removing heavy metal ions from aqueous solutions, and which are commonly practiced in the pretreatment of industrial waste-waters containing environmentally toxic metals. When dissolved heavy metal solutions are free of chelating agents, they can be effectively treated by simply admixing an alkaline or caustic compound to precipitate the insoluble metal hydroxide. Sodium hydroxide, soda ash, lime or magnesium hydroxide slurry are all used to do this. Unfortunately, such processes generate large volumes of hydroxide sludge which must be disposed of in an environmentally safe manner.
Frequently, however, complexing ammonium ions of chelating compounds such as the sodium salts of etheylenediaminetetra-acetic acid (E.D.T.A.) and others having similar properties are present. They occur as ingredients in the used plating baths, cleaners and brighteners drained into the waste-water. In such cases, it is necessary either; 1) to use a strong chemical that breaks the chelant-to-heavy metal bond and forms a stable, insoluble compound or complex of the toxic metals, or 2) to add a substance that exerts a stronger attraction for the chelant than does the toxic metal ion, to free the heavy metal to precipitate as an insoluble hydroxide. Processes of both types are currently practiced.
Sodium sulfide is used to effectively precipitate heavy metals. Its sole advantage is the extremely low solubility of most heavy metal sulfides. Most are capable of existing in the presence of even the strongest chelating agents. Undesirable aspects of using a sulfide process include the extreme toxicity of hydrogen sulfide gas which can be generated by contacting the sulfides with strong acids. Also, metal sulfide precipitates are slimy and difficult to filter. Large quantities of flocculants and filter aids are used, generating large volumes of sludge and corresponding high disposal costs.
Sodium borohydride is a strong, water soluble reducing agent that has an advantage of producing a compact semi-metallic sludge. There are several reasons for its not having broad acceptance for heavy metal removal in waste-water treatment: 1) it is very expensive, 2) precipitated metals easily reoxidize and redissolve in the presence of dissolved ammonia, 3) dangerous concentrations of potentially explosive hydrogen gas can accumulate in the space above a reaction using sodium borohydride, and 4) at times when pH is not controlled perfectly, reactions occurring at an elevated pH of 8 or higher give off toxic fumes of hydrogen sulfide gas, dangerous to workers and sensitive equipment.
Hydrazine is another strong reducing chemical capable of breaking a metal ion bond to chelants. It is used to a limited extent for heavy metal removal, but like borohydride, it too is very expensive to use and it too can generate dangerous volumes of hydrogen gas when acidified. Hydrazine has also been placed on a list of chemicals suspected of being carcinogenic. This has been a major impediment to its industrial use.
Several compounds have been used that form insoluble metal complexes with heavy metal ions. All exert a stronger attraction to the metal ion than the chelants normally occurring with the metals in the waste-waters. Insoluble starch xanthate is one such material, reportedly effective at complete removal of dissolved metal from the water. Its drawback is its generation of huge volumes of sludge, which retains a high water content and costs the user a severe penalty for disposing of same as a hazardous waste.
Other such complexing agents have gained widespread us including sodium dimethyldithiocarbamate (D.T.C.), and sodium diethyldithiocarbamate (D.E.T.C.). These are fairly effective at completely removing the heavy metal ions from solution. However, D. T.C. products require costly reclaiming in order to recycle the recovered heavy metal. The precipitate is light in density and difficult to gravity settle. The sludge often floats on water and it also gives off a foul smelling odor that is characteristic of the D.T.C. products. In addition, the dithiocarbamate compounds exhibit acute biological toxicity toward aquatic plant and animal species. Sodium dimethylidithiocarbamate is also used as the active ingredient in several EPA registered pesticide products.
At the present time, strict biological toxicity standards are being enforced upon industries by municipal sewerage authorities. Effluent toxicity is measured by placing live specimens of plant and animal species in diluted samples of such treated waste-waters. Recent data indicate that interactions exist between very low concentrations of certain heavy metals such as copper and silver, and certain anions such as nitrate, which produce more toxicity than is attributable to each component by itself. The implication of these developments is that even lower levels of removal of heavy metal ions from industrial effluents is required. A costly evaluation of background toxicity factors is required when an industry's effluent fails to meet specific toxicity limits.
All chemical methods for removing heavy metals from industrial wastes and waste-waters that are of practical use and in actual practice involve chemical reductions to metallic form and others produce metal compounds either insoluble organo-metallic complexes or metal sulfide or hydroxide sludges. The sludges of all these processes are fairly soluble in acidic water and the heavy metals are rapidly redissolved if the material is exposed to strongly acidic water.
The conventional waste-water treatment process, perhaps most frequently used by the largest number of industries, uses ferrous sulfate heptahydrate powder. Ferrous ion is substituted at a controlled acidic pH of about 2 to 3, to replace toxic heavy metal ions that are bonded by chelating agents. This allows the heavy metal ions to be rendered insoluble as hydroxides which are precipitated from an alkaline solution.
In the presence of strong chelants or free ammonia dissolved in alkaline solutions, a large excess of this source of ferrous ion is required. Normally, 5 to 10 ferrous ions are added for each copper ion being removed from chelated waste-waters. In heavily chelated streams, as many as 25 to 30 ferrous ions per heavy metal ion may be required in order to prevent the chelants from dissolving the heavy metal hydroxide. The commercial ferrous sulfate has seven waters of hydration and is only about 20% iron by weight. In some cases, over 100 pounds of ferrous sulfate powder is added to the waste-water for each pound of chelated or ammoniated copper removed, thereby generating 60 to 80 pounds of sludge.
In typical treatment systems, each additional pound of iron used adds about 4 pounds to the weight of sludge made. This can be reduced to about 3 pounds of dry sludge per pound of iron if a sludge dryer is used. When ferrous sulfate is dissolved into waste-water, it causes acidity in the water. Each mole of iron introduced this way requires using two moles of sodium hydroxide to neutralize the iron and form ferrous hydroxide. Therefore, when large excess amounts of ferrous sulfate heptahydrate powder are used, the total chemical cost for treatment is compounded. Higher hazardous waste sludge disposal costs are also incurred.
It has been proposed in U.S. Pat. No. 5,102,556 to recycle ferric hydroxide sludge which may contain occluded heavy metals in order to produce ferrous chloride. The ferrous chloride is utilized in a process known as Unipure process described in U.S. application Ser. No. 042,565 filed Apr. 16, 1987 and U.S. application Ser. No. 07/359,872 filed May 1, 1989. In this process, an aqueous solution of heavy metals and a ferrous component, e.g., ferrous chloride is rapidly oxidized, usually by air injection to produce a sludge containing the heavy metals and ferric or oxyferric hydroxide. This sludge is recycled to produce the ferrous chloride in a two step procedure. In the first step the sludge is reacted with hydrochloric acid to produce dissolved ferric chloride. In the second step, the ferric chloride is reacted with iron, usually in the form of iron powder to produce the ferrous chloride. Intermediate filtration steps can be utilized to remove undissolved species such as unreacted iron, metal chlorides or hematite. This process is inefficient and undesirable since valuable reducing capacity of metallic iron is consumed in converting ferric ion to ferrous iron rather than being consumed to reduce dissolved heavy metal ions to heavy metals precipitate that can be easily recovered. In addition, the use of an oxidation step on the heavy metal solution containing iron limits its utility for treatments of solutions containing chelated heavy metals since the presence of a strong oxidation potential enhances the attraction of the chelating agents for the heavy metals and decreases the bonding attractions between the chelating agents and the dissolved ion. As a result the tendency of the heavy metals to remain dissolved is increased. To overcome the disadvantage, the Unipure process is required to use excessively high ratios of iron to heavy metal in the water treatment process that employs such oxidation of the iron. This results in an increased consumption of iron and a net increase of the amount of process sludge produced. Typically an increase of 1 part by weight of net metallic iron results in a net increased production of about 5 to 10 parts by weight of iron bearing wet filter cake that must be disposed of.
U.S. Pat. No. 5,039,428 discloses a process for removing heavy metals from water by effecting precipitation on the surface of particles having alkaline surface activity and to produce a waste water stream. Alkaline reagent is added to a portion of the waste water to render the particle surfaces alkaline active. The activated particles then are recycled to the precipitation step. A second portion of waste water from the precipitation step is filtered to produce sludge and water, free of particles and sludge. This process does not involve a reducing step to form heavy metal particles.
It would be highly desirable to provide a safe, simple, reliable and economical process for removing heavy metals from aqueous solutions that would:
1) yield a superior quality aqueous effluent that is low in biological toxicity and compliant with all regulations for discharging into a public sewer or waterway, and, PA1 2) eliminate producing an F006 hazardous waste sludge that is normally generated at an alkaline pH and usually has a lower metal content, and PA1 3) reclaim the metals in a concentrated metallic form that yields a net positive value when recycled, and, PA1 4) use readily available and economic materials that are non-hazardous and do not cause irritating or foul odors or explosive gases.